1. Field of the Invention
The present invention relates to a mounting structure of a ball grid array, and more particularly, to the mounting structure of the ball grid array which can be favorably employed in case where a wiring board carrying the ball grid array is fixed with screws to a mounting member, such as a case body, at a plurality of points.
2. Description of the Related Art
The ball grid array (hereinafter referred to as the “BGA”) is provided with terminals and solder balls which are arranged on its lower face. When the BGA is mounted on a wiring board, the solder balls will be melted thereby to connect the terminals to a conductor pattern on the wiring board. For this reason, there has been such anxiety that when an irregular warping has occurred in the wiring board on which the BGA had been mounted, the BGA will float up from the wiring board, and connecting points between the terminals and the aforesaid conductor pattern may be peeled off or broken. As reasons for causing the floating of the BGA due to the warping of the wiring board, a warping which has already occurred in the wiring board itself, and besides, an irregular distortion which may occur when the wiring board is fixed to the mounting member with small screws are considered. Although the former should be dealt with by enhancing manufacturing accuracy of the wiring board and so on, the latter cannot be necessarily dealt with by enhancing the manufacturing accuracy of the wiring board.
Such floating of the BGA from the wiring board attributed to the irregular distortion which occurs when the wiring board is fixed with the screws to the mounting member, such as a case body, will be described, referring to a conventional example which is shown in FIGS. 6A to 6C, 7, and 8.
FIG. 6A is a schematic plan view showing a conventional mounting structure of a ball grid array, FIG. 6B is a schematic view as seen in a direction of VIB in FIG. 6A, and FIG. 6C is a schematic view as seen in a direction of VIC in FIG. 6A. FIG. 7 is an enlarged view of an essential part of a wiring board 1 in FIGS. 6A to 6C, and FIG. 8 is an enlarged sectional view showing a point P where the wiring board 1 of FIGS. 6A to 6C is fixed to a mounting member 5, such as a case body, with a screw.
In the conventional mounting structure of the ball grid array, a BGA 2 is mounted in a center part or in a place close to the center part both in a vertical direction a and in a lateral direction b of the wiring board 1 which has, for example, a rectangular shape, as shown in FIG. 6A. Four screw insertion holes 12 for fixing the wiring board with screws are positioned in respective four corners of an imaginary rectangular outline L1 which is imagined on a board surface of the wiring board 1. Besides, an entirety of the wiring board 1 is divided into a first zone Z1 in a rectangular shape which is surrounded by all the screw insertion holes 12 positioned at the four points P to be fixed with the screws, and a second zone Z2 which is formed by an area S positioned at one side of the first zone Z1 in the lateral direction b. An entirety of mounting position of the BGA2 is contained in the first zone Z1. On the other hand, as shown in FIGS. 6A to 6C or FIG. 7, on a back face of the wiring board 1, there are formed solder banks 13 as ground electrodes which are adapted to be pressure contacted with a ground connecting face (described below) provided on the mounting member 5 by fixing with screws at a plurality of positions around the respective screw insertion holes 12. As shown in FIG. 8, in a state where the wiring board 1 has been fixed to the mounting member 5, a fitting screw 14 which has been passed through the screw insertion hole 12 is screwed into a screw hole 51 of the mounting member 5, and at the same time, the aforesaid solder banks 13 as the ground electrodes are pressure contacted with a ground connecting face 52 which is provided on the mounting member 5 through tightening action of this fitting screw 14. Because of this pressure contact between the solder banks 13 and the ground connecting face 52, grounding stability of the wiring board 1 is enhanced. Although the mounting member 5 as shown in FIG. 8 is divided into an outer casing as a case body, and a chassis made of sheet metal to be grounded, it is also probable that this mounting member 5 may be formed only by the case body made of sheet metal.
In a state where the wiring board 1 has been fixed with the screws to the mounting member 5 at the four points in this manner, influence of tightening loads of the fitting screws 14 which have been applied to the four screwing points P will be exerted on the wiring board 1, and a warping may occur in the first zone Z1 of the wiring board which is defined by the four screwing points, even though the wiring board has been manufactured with high precision so as not to have a warping in itself. For this reason, such phenomenon that the wiring board 1 is irregularly distorted in the first zone Z1 in two directions perpendicular to each other, for example, may sometimes happen. When the wiring board 1 has an irregular warping in the first zone Z1 of the wiring board 1, there will occur floating of the BGA 2 mounted on the first zone Z1 from the wiring board 1. FIGS. 6B and 6C illustratively show the wiring board 1 in a state where the wiring board 1 has been irregularly distorted, and the warping has occurred in the two directions perpendicular to each other in the first zone Z1. When the above described floating of the BGA has occurred as shown in the drawings, there will be such anxiety that a gap is formed in a position A where the BGA 2 is overlapped on the wiring board 1, and the connecting points between the terminals of the BGA and the conductor pattern of the wiring board 1 may be peeled off or broken.
In case where the solder banks 13 as the ground electrodes are formed around the respective screw insertion holes 12 at the four points, as in the above described wiring board 1, the floating of the BGA 2 attributed to the irregular warping which has occurred in the first zone Z1 of the wiring board 1 will be more prominently promoted because of variations in height of the solder banks 13 or so.
However, it has been difficult to visually detect the floating with naked eyes, because a width of the warping in the wiring board 1 when it has been irregularly warped and a width of the floating of the BGA 2 when it has floated up from the wiring board 1 have been extremely small. As the results, there has been a problem that defective boards in which incomplete connection of the BGA has happened may occur.
On the other hand, an art of reinforcing the wiring board in order to restrain the wiring board from being warped has been heretofore proposed (Reference should be made to JP-A-2000-58703 and JP-A-11-163043). Moreover, an art of disposing hard glass bodies for the purpose of detecting that a warping has occurred in the wiring board, around the mounting position of the BGA on the wiring board has been also proposed (Reference should be made to JP-A-10-256693).